Quantum devices, such as dots, wires and wells, rely upon the quantization of energy into discrete energy levels. If electrons become trapped into a structure with at least one reduced dimension, the density of electron states, and the energy levels that electrons can occupy, become quantized. Quantum dots may be formed by thin-film deposition techniques including molecular beam epitaxy (MBE) and chemical vapor deposition (CVD). In order to lengthen the electron-hole pair lifetime in the system consisting of the quantum dot and its substrate, relative to the bulk material, a material system may separate photo-excited electrons and holes by virtue of it's electronic structure. The essential property is that the energies of the valence and conduction band edges in one material must be concomitantly lower or higher than the other material, which is the definition of a type II heterostructure.
The separation of electrons and holes provides relatively long-lived electrons and holes useful in a number of applications including photocatalysis. These electrons and holes can be transferred to molecules or ions at the surface, causing an oxidation-reduction (redox) reaction. Typical photocatalytic applications include energy production and removal of organic pollutants.
In photocatalytic applications, the use of oxides is relatively common. Oxide materials are stable and promote the redox reactions desired. However, no known photocatalytic applications use the unique characteristics of oxide quantum dots grown on crystalline oxide substrates. These include the separation of charge carriers brought about by the presence of a type II heterojunction and the presence of these carriers on the same side of the material.
One common problem with the current state of the art is the relatively short lifetime of photoexcited electron-hole pairs. The speed at which the electrons and holes recombine limits the availability of the separate electrons and holes. In order to be useful for many applications, such as photocatalysis as well as photodetection and other applications, the recombination time must be longer than the heterogeneous electron transfer reaction rate. Spatial separation can increase this time, but current embodiments of quantum dot formation do not provide sufficient spatial separation.
Therefore, while the use of oxides in thin films may be known, as is the growth of quantum dots, no current material system exists that use oxide quantum dots.